U.S. patent application number 13/581743 was filed with the patent office on 2013-01-03 for photonic crystal laser.
This patent application is currently assigned to KYOTO UNIVERSITY. Invention is credited to Kyoko Kitamura, Yoshitaka Kurosaka, Susumu Noda, Kyosuke Sakai.
Application Number | 20130003768 13/581743 |
Document ID | / |
Family ID | 44542162 |
Filed Date | 2013-01-03 |
United States Patent
Application |
20130003768 |
Kind Code |
A1 |
Noda; Susumu ; et
al. |
January 3, 2013 |
PHOTONIC CRYSTAL LASER
Abstract
A photonic crystal laser capable of producing a
radially-polarized halo-shaped laser beam having a smaller width
than conventional beams includes: an active layer; a ring-shaped
photonic crystal including a plate-shaped base body on one side of
the active layer, the base body having a number of modified
refractive index areas of the same shape, the modified refractive
index areas having a refractive index different from the base body
and periodically arranged in the circumferential direction of a
ring, and each of the modified refractive index areas being
asymmetrically shaped with respect to an axis extending through the
center of the modified refractive index area in the radial
direction of the ring; a first and second electrode facing each
other across the active layer and the ring-shaped photonic crystal;
and a window provided in the second electrode capable of allowing
passage of a laser light generated from the ring-shaped photonic
crystal.
Inventors: |
Noda; Susumu; (Kyoto-shi,
JP) ; Kitamura; Kyoko; (Kyoto-shi, JP) ;
Kurosaka; Yoshitaka; (Hamamatsu-shi, JP) ; Sakai;
Kyosuke; (Sapporo-shi, JP) |
Assignee: |
KYOTO UNIVERSITY
Kyoto-shi, Kyoto
JP
|
Family ID: |
44542162 |
Appl. No.: |
13/581743 |
Filed: |
March 1, 2011 |
PCT Filed: |
March 1, 2011 |
PCT NO: |
PCT/JP2011/054566 |
371 Date: |
August 29, 2012 |
Current U.S.
Class: |
372/41 |
Current CPC
Class: |
H01S 5/105 20130101;
B82Y 20/00 20130101; H01S 2301/203 20130101; H01S 5/0267 20130101;
H01S 5/34333 20130101; H01S 2301/14 20130101; H01S 5/1071 20130101;
H01S 5/187 20130101 |
Class at
Publication: |
372/41 |
International
Class: |
H01S 3/16 20060101
H01S003/16 |
Foreign Application Data
Date |
Code |
Application Number |
Mar 1, 2010 |
JP |
2010-044255 |
Claims
1-10. (canceled)
11. A photonic crystal laser, comprising: a) an active layer; b) a
ring-shaped photonic crystal including a plate-shaped base body on
one side of the active layer, the base body having a number of
modified refractive index areas of a same shape, the modified
refractive index areas having a refractive index different from
that of the base body and being periodically arranged in a
circumferential direction of a ring, and each of the modified
refractive index areas being asymmetrically shaped with respect to
an axis extending through a center of the modified refractive index
area in a radial direction of the ring; c) a first electrode and a
second electrode facing each other across the active layer and the
ring-shaped photonic crystal; and d) a window provided in the
second electrode, the window being capable of allowing passage of a
laser light generated from the ring-shaped photonic crystal.
12. The photonic crystal laser according to claim 11, wherein the
first electrode is a ring-shaped electrode whose diameter and width
overlap those of the ring-shaped photonic crystal.
13. The photonic crystal laser according to claim 12, wherein a
current-narrowing portion having a current-passing area being
identical in shape to the ring-shaped electrode and an insulating
area covering areas around the current-passing area are provided
between the ring-shaped electrode and the ring-shaped photonic
crystal.
14. The photonic crystal laser according to claim 11, comprising a
ring-shaped groove on each of inner and outer sides of the
ring-shaped photonic crystal.
15. The photonic crystal laser according to claim 11, wherein a
ring-shaped convex lens bulging in a direction from the ring-shaped
photonic crystal toward the second electrode is provided at the
window of the second electrode.
16. The photonic crystal laser according to claim 11, wherein an
outer circumferential width, which is a width of the modified
refractive index area on an outer circumference of the ring-shaped
photonic crystal is different from an inner circumferential width,
which is a width of the modified refractive index area on an inner
circumference of the ring-shaped photonic crystal.
17. The photonic crystal laser according to claim 16, wherein the
outer circumferential width is larger than the inner
circumferential width and smaller than
(r.sub.2.sup.2/r.sub.1.sup.2) times the inner circumferential
width, where r.sub.1 is an inner diameter of the ring-shaped
photonic crystal and r.sub.2 is an outer diameter of the
ring-shaped photonic crystal.
18. The photonic crystal laser according to claim 17, wherein the
outer circumferential width is (r.sub.2/r.sub.1) times the inner
circumferential width.
19. The photonic crystal laser according to claim 11, wherein each
of the modified refractive index areas is composed of a main
modified refractive index area and a sub modified refractive index
area separated from the main modified refractive index area by a
predetermined distance in the circumferential direction of the
ring, the sub modified refractive index area having a refractive
index different from that of the base body and differing from the
main modified refractive index area at least in term of area, shape
or refractive index.
20. The photonic crystal laser according to claim 19, wherein the
distance between the main modified refractive index area and the
sub modified refractive index area is one fourth of a cycle
distance of the modified refractive index areas.
Description
TECHNICAL FIELD
[0001] The present invention relates to a photonic crystal laser
suitable as a light source for fluorescence microscopes, Raman
spectrometers or similar devices.
BACKGROUND ART
[0002] The light source used in a fluorescence microscope, Raman
spectrometer or similar optical measurement device is required to
focus a laser beam into the smallest possible spot in order to
improve the measurement accuracy. However, when the laser beam is
simply focused by a focus lens, the spot size of the laser beam
cannot be smaller than the diffraction limitation which is
determined by the wavelength of the laser beam and the numerical
aperture of the focus lens.
[0003] Non-Patent Document 1 discloses a laser beam suitable for
reducing the spot size. FIG. 21A schematically shows a cross
section of this laser beam at a plane perpendicular to the beam
(this cross section hereinafter will be simply referred to as the
"section of the laser beam"). The shaded portion in this figure
corresponds to the area where the beam is present. The thick arrows
in the figure indicate the polarizing direction. The laser beam has
a ring-shaped cross section having no intensity in the central
region and is polarized in the direction from its center toward the
outer area (i.e. in the radial direction). A laser beam having such
a cross-sectional shape and polarization is hereinafter referred to
as the "radially-polarized ring-shaped laser beam." According to
Non-Patent Document 1, the radially-polarized ring-shaped laser
beam 21 can be focused into a laser beam whose spot size is smaller
than the diffraction limitation. Conventionally, such a small spot
could only be obtained over an extremely short range of up to
approximately one wavelength in the direction of the optical axis.
By using the new laser beam, it is possible to obtain a small spot
over a range equal to or even longer than ten times the wavelengths
(see Non-Patent Document 2).
[0004] As shown in FIG. 21B, when the radially-polarized
ring-shaped laser beam 21 is focused by a lens 23, the
electric-field components oscillating in the directions
perpendicular to the laser beam (in the x and y directions) at the
condensing point 22 are cancelled out, while the electric-field
component oscillating in the direction parallel to the laser beam
(in the z direction) remains without being cancelled out. As a
result, at the condensing point 22, an oscillation of the electric
field in the direction parallel to the travelling direction of the
beam is created over a wide range of approximately ten times the
wavelengths on the beam axis (see Non-Patent Document 2), Such a
polarization (i.e. z-polarization) does not occur on the beam axis
when a normal beam is used. Using the z-polarization is
advantageous in some cases; for example, in a Raman scattering
measurement, it is possible to observe a scattering that does not
occur when a polarization perpendicular to the laser beam is used
(see Non-Patent Document 3).
BACKGROUND ART DOCUMENT
Non-Patent Document
[0005] Non-Patent Document 1: S. Quabis et al., "Focusing light to
a tighter spot", Optics Communications, vol. 179 (2000), pp. 1-7
[0006] Non-Patent Document 2: K. Kitamura et al., Optics Express,
vol. 18 (2010), iss. 5, pp. 4518-4525 [0007] Non-Patent Document 3:
"Z-Henkou Soshi (Z-Polarizing Element)", [online], 2005, Nanophoton
Corporation, [Searched on Feb. 22, 2010], Internet <URL:
http://www.nanophoton.jp/products/zpol/index.html> [0008]
Non-Patent Document 4: H. Kogelnik et al., Journal of Applied
Physics, vol. 43 (1972), pp. 2327-2335
SUMMARY OF THE INVENTION
Problem to be Solved by the Invention
[0009] To further reduce the spot size of the laser beam, it is
preferable to reduce the width of the ring in the
radially-polarized ring-shaped laser beam. The problem to be solved
by the present invention is to provide a photonic crystal laser
capable of producing a radially-polarized halo-shaped laser beam
having a smaller width than that of the conventional beam.
Means for Solving the Problems
[0010] A photonic crystal laser according to the present invention
aimed at solving the aforementioned problem includes:
[0011] a) an active layer;
[0012] b) a ring-shaped photonic crystal including a plate-shaped
base body on one side of the active layer, the base body having a
number of modified refractive index areas of the same shape, the
modified refractive index areas having a refractive index different
from that of the base body and being periodically arranged in the
circumferential direction of a ring, and each of the modified
refractive index areas being asymmetrically shaped with respect to
an axis extending through the center of the modified refractive
index area in the radial direction of the ring;
[0013] c) a first electrode and a second electrode facing each
other across the active layer and the ring-shaped photonic crystal;
and
[0014] d) a window provided in the second electrode, the window
being capable of allowing passage of a laser light generated from
the ring-shaped photonic crystal.
[0015] In the photonic crystal laser according to the present
invention, a voltage is applied between the first and second
electrodes to inject electric current into the active layer,
whereby light is generated within the active layer, and this light
is introduced into the ring-shaped photonic crystal. Within the
ring-shaped photonic crystal, a component of the introduced light
having a specific wavelength corresponding to the cycle distance of
the periodic structure of the ring-shaped photonic crystal is
selectively amplified due to interference, causing laser
oscillation. The laser light generated in the ring-shaped photonic
crystal is emitted in the direction perpendicular to the plate of
the base body, to be extracted through the window of the second
electrode to the outside.
[0016] The laser light has a halo-shaped cross section
corresponding to the shape of the ring-shaped photonic crystal.
Accordingly, it is possible to decrease the difference between the
inner and outer diameters of the halo on the cross section of the
beam, i.e. to reduce the width of the halo, by decreasing the
difference between the inner and outer diameters of the ring-shaped
photonic crystal. Since each modified refractive index area is
asymmetrically shaped with respect to the axis extending through
the center thereof in the radial direction of the ring, the
polarization of the laser beam will be in the radial direction
along this axis. Thus, the photonic crystal laser according to the
present invention produces a radially-polarized halo-shaped laser
beam.
[0017] A ring-shaped electrode whose diameter and width overlap
those of the ring-shaped photonic crystal is preferable as the
first electrode. By this design, the light generated within the
active layer can be efficiently introduced into the ring-shaped
photonic crystal since electric current is intensively injected
into a region of the active layer near the ring of the ring-shaped
photonic crystal. In this case, a current-narrowing portion having
a current-passing area being identical in shape to the ring-shaped
electrode and an insulating area covering the areas around the
current-passing area may preferably be provided between the
ring-shaped electrode and the ring-shaped photonic crystal. By this
design, the electric current can be more intensively injected into
the aforementioned region of the active layer.
[0018] In the photonic crystal laser according to the present
invention, a ring-shaped groove may be provided on each of the
inner and outer sides of the ring-shaped photonic crystal. These
grooves also contribute to the intensive injection of electric
current into the region near the ring of the ring-shaped photonic
crystal.
[0019] A ring-shaped convex lens bulging in the direction from the
ring-shaped photonic crystal toward the second electrode may be
provided at the window of the second electrode. This lens has the
effect of reducing the width of the halo on the cross section of
the laser light.
[0020] One example of the window provided in the second electrode
is a plate-shaped member made of a material of the electrode with a
central portion cut out. In this case, the cut-out area serves as
the window, and the remaining portion of the electrode material
serves as the second electrode. It is also possible to make the
second electrode of a material transparent to the generated laser
light, in which case the entirety of the second electrode serves as
the window.
[0021] The width of the modified refractive index area on the outer
circumference of the ring-shaped photonic crystal (which is
hereinafter referred to as the "outer circumferential width") may
be different from the width of the modified refractive index area
on the inner circumference of the ring-shaped photonic crystal
("inner circumferential width").
[0022] In this case, the intensity of the laser beam at a cross
section will be as follows: When the permittivity of the modified
refractive index areas is lower than that of the base body (e.g.
when the modified refractive index areas are air holes), the
emission of the laser beam at a cross section on the outer
circumference will be stronger if the outer circumferential width
is smaller than the inner circumferential width, whereas the
emission on the inner circumference will be stronger if the outer
circumferential width is larger than (r.sub.2.sup.2/r.sub.1.sup.2)
times the inner circumferential width (where r.sub.1 and r.sub.2
are respectively the inner and outer diameters of the ring-shaped
photonic crystal). These patterns will be reversed when the
permittivity of the modified refractive index areas is higher than
that of the base body. Additionally, when the outer circumferential
width is larger than the inner circumferential width and smaller
than (r.sub.2.sup.2/r.sub.1.sup.2) times the latter width, the
intensity of light at the cross section of the laser beam will be
closer to the state of uniformity than when the outer
circumferential width is equal to the inner circumferential width,
regardless of whether the permittivity of the modified refractive
index areas is higher or lower than that of the base body. The
reason will be hereinafter explained.
[0023] The explanation initially concerns the case where the
permittivity of the modified refractive index areas is lower than
that of the base body. If the outer circumferential width is equal
to the inner circumferential width, the density of the modified
refractive index areas (filing factor: the ratio of the area
occupied by the modified refractive index areas in the photonic
crystal) on the outer circumference of the ring is
(r.sub.1/r.sub.2) times the density on the inner circumference.
This means that the density of the modified refractive index areas
is lower on the outer circumference than on the inner
circumference. Accordingly, the effective permittivity is higher on
the outer circumference of the ring-shaped photonic crystal than on
the inner circumference thereof. The electric field of the light
introduced into the ring-shaped photonic crystal is likely to be
concentrated on the region having a higher effective permittivity
within the crystal. Therefore, if the effective permittivity is
distributed in the previously described form, the emission of the
laser light at a cross section will be stronger on the outer region
of the ring than on the inner region. Accordingly, if the outer
circumferential width is made to be larger than the inner
circumferential width and smaller than
(r.sub.2.sup.2/r.sub.1.sup.2) times the latter width, the
distribution of the effective permittivity will be closer to the
state of uniformity than when the outer circumferential width is
equal to the inner circumferential width, so that the intensity of
light at the cross section of the laser light will also be closer
to the state of uniformity. By contrast, if the outer
circumferential width is smaller than the inner circumferential
width, the effective permittivity on the outer circumference will
be even higher, causing the emission of the laser light at the
cross section to be even stronger on the outer region of the ring.
If the outer circumferential width is larger than
(r.sub.2.sup.2/r.sub.1.sup.2) times the inner circumferential
width, the effective permittivity will be higher on the inner
circumference of the ring-shaped photonic crystal than on the outer
circumference thereof; so that the emission of the laser light at
the cross section will be stronger on the outer region of the ring
than on the inner region thereof.
[0024] In the case where the permittivity of the modified
refractive index areas is higher than that of the base body, the
relationship in the magnitude of the effective permittivity will be
opposite to the previously described case, so that the relationship
in the magnitude of the intensity of light at the cross section of
the laser light will also be opposite.
[0025] If the outer circumferential width is (r.sub.2/r.sub.1)
times the inner circumferential width, the density of the modified
refractive index areas on the inner circumference of the
ring-shaped photonic crystal will be equal to the density on the
outer circumference, so that the intensity of light at the cross
section of the laser light will be closest to the state of
uniformity.
[0026] Each of the modified refractive index areas may be composed
of a main modified refractive index area and a sub modified
refractive index area separated from the main modified refractive
index area by a predetermined distance in the radial direction of
the ring, the sub modified refractive index area having a
refractive index different from that of the base body and differing
from the main modified refractive index area at least in term of
area, shape or refractive index.
[0027] When such a sub modified refractive index area is used,
interference of the light reflected (diffracted) by the main
modified refractive index area (main reflection) and the light
reflected (diffracted) by the sub modified refractive index area
(sub reflection) occurs in the ring-shaped photonic crystal. This
interference causes the light to strengthen or weaken depending on
the distance .delta. between the main modified refractive index
area and the sub modified refractive index area. In order to
strengthen the laser light, the distance .delta. can be set so that
the main and sub reflections will constructively interfere with
each other.
[0028] However, if the intensity of the diffracted light per unit
length (optical coupling coefficient .kappa.) within the photonic
crystal (resonator) increases, the emission is likely to
intensively occur at a portion of the photonic crystal. Photonic
crystal lasers are one type of one-dimensional distributed feedback
lasers, and it is known that this phenomenon generally occurs in
one-dimensional distributed feedback lasers (Non-Patent Document
4). This leads to a difference in the intensity of light at the
cross section of the laser light. A constructive interference of
the main and sub reflections further increases this difference in
the intensity of the obtained light, making it impossible to obtain
a laser light having a halo-shaped cross section with a uniform
intensity distribution. Accordingly, to obtain a laser light having
a halo-shaped cross section with an approximately uniform intensity
distribution, the distance .delta. can be appropriately set so that
the main and sub reflections destructively interfere with each
other. More specifically, the distance .delta. should preferably be
one fourth of the cycle distance of the modified refractive index
areas.
Effect of the Invention
[0029] By the present invention, it is possible to obtain a
photonic crystal laser which oscillates a radially-polarized
halo-shaped laser beam having a halo-shaped cross section and being
polarized in the radial direction of the halo. The width of the
halo of the laser beam can be decreased by reducing the width of
the ring-shaped photonic crystal, whereby a radially-polarized
halo-shaped laser beam having a smaller width than that of the
conventional beam can be obtained. A device obtained by combining
the present photonic crystal laser with a lens for focusing the
obtained laser beam can be used as a light source capable of
producing a beam having a small spot size and z-polarization, the
small spot shape and z-polarization being created over a wide range
on the beam axis. Such a light source can suitably be used in
fluorescence microscopes, Raman spectrometers or other measurement
devices.
BRIEF DESCRIPTION OF THE DRAWINGS
[0030] FIG. 1A is a vertical sectional view of a photonic crystal
laser according to one embodiment of the present invention, FIG. 1B
is a plan view of the ring-shaped photonic crystal, FIG. 1C is a
plan view of the first electrode, and FIG. 1D is a plan view of the
second electrode.
[0031] FIG. 2 is a partially enlarged view of the ring-shaped
photonic crystal in the photonic crystal laser of the present
embodiment.
[0032] FIG. 3 is a vertical sectional view of one example of the
photonic crystal laser of the present embodiment provided with a
current-narrowing portion.
[0033] FIGS. 4A and 4B are a vertical view and plan view of one
example of the photonic crystal laser of the present embodiment
provided with a ring-shaped lens.
[0034] FIGS. 5A-5C are plan views showing examples of the modified
refractive index areas in the ring-shaped photonic crystal in the
photonic crystal laser of the present embodiment.
[0035] FIGS. 6A-6C are plan views showing examples of the modified
refractive index areas arranged at varying intervals in the radial
direction.
[0036] FIGS. 7A-7C are plan views showing examples of the modified
refractive index areas arranged at varying intervals in the radial
direction and with changing shapes.
[0037] FIGS. 8A-8C are plan views showing examples of the modified
refractive index areas arranged in the radial direction at
intervals different from the intervals in the circumferential
direction.
[0038] FIGS. 9A-9H are plan views showing examples of the modified
refractive index areas each of which has a shape created by
combining one of the shapes of FIGS. 5A-8C with a thin rectangle
extending in the radial direction.
[0039] FIG. 10A is an electron microscope image of a ring-shaped
photonic crystal in a photonic crystal laser created in the present
embodiment, and FIG. 10B is an electron microscope image showing an
enlarged view of the modified refractive index areas.
[0040] FIG. 11 is a graph showing an oscillation spectrum of the
photonic crystal laser created in the present embodiment.
[0041] FIGS. 12A and 12B are a near-field image and far-field image
of a laser beam produced by the photonic crystal laser created in
the present embodiment.
[0042] FIGS. 13A-13D are photographs showing the result of a
measurement of the polarizing direction of a laser beam produced by
the photonic crystal laser created in the present embodiment.
[0043] FIG. 14 is a plan view showing one example of the modified
refractive index area with the inner and outer circumferential
widths differing from each other.
[0044] FIGS. 15A-15D are plan views each showing other examples of
the modified refractive index areas with the inner and outer
circumferential widths differing from each other.
[0045] FIG. 16 is a plan view showing one example of the modified
refractive index area composed of a main modified refractive index
area and a sub modified refractive index area.
[0046] FIG. 17 is an electron microscope image showing a portion of
a ring-shaped photonic crystal using a modified refractive index
area composed of a main modified refractive index area and a sub
modified refractive index area.
[0047] FIG. 18 is a diagram illustrating the interference of light
which occurs when the sub modified refractive index area is
present.
[0048] FIGS. 19A and 19B show the result of a calculation of the
magnitude of the optical coupling coefficient .kappa. for a device
using a modified refractive index area consisting of only the main
modified refractive index area (FIG. 19A) and for a device using a
modified refractive index area composed of a main modified
refractive index area and a sub modified refractive index area
(FIG. 19B).
[0049] FIG. 20 is a plan view showing other examples of the
modified refractive index area composed of a main modified
refractive index area and a sub modified refractive index area.
[0050] FIG. 21A is a sectional view of a radially-polarized
halo-shaped laser beam at a plane perpendicular to the beam, and
FIG. 21B is a sectional view of the same beam parallel to the
beam.
BEST MODE FOR CARRYING OUT THE INVENTION
[0051] Embodiments of the photonic crystal laser according to the
present invention are hereinafter described by means of FIGS.
1A-20.
Embodiments
[0052] As shown in FIG. 1A, the photonic crystal laser 10 according
to the present embodiment includes an active layer 11, a photonic
crystal layer 12 provided on one side of the active layer 11, as
well as a lower electrode (first electrode) layer 13 and an upper
electrode (second electrode) layer 14 facing each other across the
first two layers. In the present case, the lower electrode layer 13
is provided on the side facing the photonic crystal layer 12, while
the upper electrode layer 14 is provided on the side facing the
active layer 11. It is also possible to provide the upper electrode
layer 14 on the side facing the photonic crystal 12 and the lower
electrode layer 13 on the side facing the active layer 11.
[0053] The active layer 11 may be any type of active layer commonly
used in conventional semiconductor lasers. In the present
embodiment, a material having a multiple-quantum well (MQW) made of
Indium Gallium Arsenic (InGaAs)/Gallium Arsenic (GaAs) is used as
the material of the active layer 11. This active layer emits light
within a wavelength range from 0.9 to 1.1 .mu.m (infrared region).
The materials available for the active layer are not limited to the
aforementioned one; the material can be appropriately selected
according to the wavelength of the laser beam to be generated. For
example, a material suitable for a blue laser is a semiconductor
having a multiple-quantum well made of Indium Gallium Nitride
(InGaN)/Gallium Nitride (GaN), which emits light within a
wavelength range from 0.4 to 0.6 .mu.m.
[0054] As shown in FIG. 1B, the photonic crystal layer 12 consists
of a plate-shaped base body 121 in which air holes (modified
refractive index areas) 122 are periodically arranged in the form
of a ring. The ring-shaped area with the air holes 122 arranged
therein is hereinafter referred to as the "ring-shaped photonic
crystal 123." The use of air holes as the modified refractive index
areas is advantageous in that it merely requires the easy machine
work of boring holes in the base body. Alternatively, a solid
member having a refractive index different from that of the
material of the base body may be embedded as the modified
refractive index area. Embedding a member different from the base
body is advantageous in that the modified refractive index area
will be less likely to be deformed than in the case of using the
air hole. In the present embodiment, p-type GaAs is used as the
material of the base body 121. The radius of the ring is 160 .mu.m.
The distance between the neighboring air holes is 296 nm, which
corresponds to the wavelength of light in the medium. If the active
layer is made of InGaN/GaN, a GaN-type material should be used as
the material of the base body 121, with the distance between the
neighboring air holes being set at 186 nm, which corresponds to the
wavelength of light in the medium. A first ring-shaped groove 1241
is provided on the outside of the ring-shaped photonic crystal 123,
while a second ring-shaped groove 1242 is provided on the inside of
the ring-shaped photonic crystal 123.
[0055] The planer shape of the air holes 122 is hereinafter
described by means of FIG. 2. Each of the air holes 122 is
asymmetrically shaped with respect to a radial axis 129 passing
through the center of the hole and extends in the radial direction
of the ring. Accordingly, if the air holes 122 are virtually
rotated around the center O of the ring of the ring-shaped photonic
crystal 123, each air hole 122 will overlap another air hole 122.
The reason for shaping the air holes 122 in this way will be
described later.
[0056] The lower electrode layer 13 consists of a ring-shaped lower
electrode (first electrode) 131 made of an electrically conductive
material and a first insulating film 132 inside and outside the
ring. This ring of the lower electrode 131 has approximately the
same diameter as the ring-shaped photonic crystal 123.
[0057] The upper electrode layer 14 consists of an upper electrode
(second electrode) 141, which is made of an electrically conductive
material and shaped like a square frame, and a second insulating
film 142 inside and outside the upper electrode 141. The second
insulating film is made of SiN and transparent to the light within
a wavelength range including 980 to 990 nm. Accordingly, the area
inside upper electrode 141 functions as a window 143.
[0058] A p-type cladding layer 151 made of a p-type semiconductor
is provided between the photonic crystal layer 12 and the lower
electrode layer 13, while an n-type cladding layer 152 made of an
n-type semiconductor is provided between the active layer 11 and
the upper electrode layer 14. These cladding layers are also used
in conventional photonic crystal lasers. Additionally, a spacer
layer may be inserted between each neighboring pair of the
previously mentioned layers.
[0059] An operation of the photonic crystal laser 10 of the present
embodiment is hereinafter described. An electric current is
injected into the active layer 11 by applying a voltage between the
upper and lower electrodes 131 and 141. As a result, light is
emitted from the active layer 11 within a wavelength range
determined by the kind of material of this layer. The emitted light
is introduced into the ring-shaped photonic crystal 123. Since the
lower electrode 131 is a ring having approximately the same
diameter as the ring-shaped photonic crystal 123, the electric
current injected into the active layer 11 in the previously
described manner is concentrated into a region immediately above
the ring-shaped photonic crystal 123, causing a stronger emission
of light in that region than in the surrounding areas, so that the
generated light will efficiently enter the ring-shaped photonic
crystal 123.
[0060] Within the ring-shaped photonic crystal 123, a portion of
the introduced light having a specific wavelength determined by the
cycle distance of the air holes 122 is selectively amplified due to
interference, causing a laser oscillation. The generated laser
light is emitted in the direction perpendicular to the photonic
crystal layer 12, and a laser beam is extracted through the window
143 of the upper electrode 141 to the outside. As shown in FIGS.
21A and 21B, the laser beam 21 has a halo-shaped cross section
corresponding to the shape of the ring-shaped photonic crystal 123
and is polarized in the radial direction on the halo-shaped cross
section (as shown by the arrows in FIGS. 21A and 21B). The reason
for such polarization is as follows: For each air hole 122, an
oscillating electric field is created around the air hole 122 along
the boundary between this air hole 122 and the base body 121. Since
the air hole 122 is asymmetrically shaped with respect to the
radial axis 129, it has different values of effective permittivity
on the two sides of the radial axis 129, and accordingly, has
different values of intensity of the oscillating electric field.
Therefore, the electric field oscillating in the radial direction
cannot be completely cancelled out between the two neighboring air
holes 122 (FIG. 2), so that the laser beam has radial polarization
at a cross section thereof.
[0061] Variations of the photonic crystal laser 10 of the present
embodiment are hereinafter described. The photonic crystal laser
10A shown in FIG. 3 has a current-passing portion 31, which has the
same shape as the ring-shaped lower electrode 131A and is located
directly under the ring-shaped photonic crystal 123, as well as an
insulating film 32 covering the areas surrounding the
current-passing portion 31. The structural elements located on the
side closer to the upper electrode 141 from the ring-shaped
photonic crystal 123 are the same as those of the previously
described photonic crystal laser 10. The provision of the
current-passing portion 31 has the effect of narrowing the channel
of the electric current between the lower electrode 131A and the
ring-shaped photonic crystal 123, so that the electric current can
be efficiently supplied into the region of the active layer 11
directly above the two-dimensional photonic crystal 123.
[0062] The photonic crystal laser 10B shown in FIGS. 4A and 4B has
a ring-shaped lens 41 located directly above the ring-shaped
photonic crystal 123 within the area of the window 143. The
ring-shaped lens 41 is a convex lens bulging in the direction from
the ring-shaped photonic crystal 123 toward the upper electrode
layer 1413. The ring-shaped lens 41 has the effect of decreasing
the width of the halo at the cross section of the laser beam
emitted from the ring-shaped photonic crystal 123.
[0063] Subsequently, examples of the shapes of the modified
refractive index areas (which are either air holes or members whose
refractive index differ from that of the base body) that can be
used in any of the previous embodiments are described by means of
FIGS. 5A-9H. In any of these figures, the shaded areas represent
the modified refractive index areas. The horizontal direction in
the figures corresponds to the radial direction of the ring-shaped
photonic crystal 123, while the vertical direction corresponds to
the circumferential direction. In any of these examples, the
modified refractive index areas are equally spaced in the
circumferential direction with cycle distance .alpha..
[0064] The example shown in FIG. 5A is a triangular area with one
side extending parallel to the radial direction. The example shown
in FIG. 5B is a V-shaped area with its lateral direction
corresponding to the radial direction. The example shown in FIG. 5C
is a semielliptical area with the chord extending parallel to the
radial direction. Any of these examples is asymmetrically shaped
with respect to the radial axis. The triangular example, which is
an isosceles triangle in FIG. 5A, may be changed to an equilateral
triangle or a triangle with three sides having different lengths.
The semielliptical example shown in FIG. 5C may be changed to a
semicircular shape.
[0065] In any of the examples shown in FIGS. 6A-8C, a plurality of
modified refractive index areas of the same kind of shape are
arranged not only in the circumferential direction but also in the
radial direction. In the examples shown in FIGS. 6A-6C, the
modified refractive index areas are arranged at varying intervals
in the radial direction and at equal intervals in the
circumferential direction. In the examples shown in FIG. 7A-7C, the
modified refractive index areas not only change their spatial
intervals but also vary in shape in the radial direction, while
they have the same shape and spatial interval in the
circumferential direction. In the examples shown in FIG. 8A-8C, the
modified refractive index areas are equally spaced in the radial
direction at regular intervals which differ from the intervals in
the circumferential direction. In any of the examples of FIGS.
6A-8C, each individual modified refractive index area may have a
triangular, semicircular (semielliptical), V-shaped or any other
from. From the viewpoint of the periodic structure of the
ring-shaped photonic crystal, each row of the modified refractive
index areas arranged in the radial direction functions as one
modified refractive index area.
[0066] In the examples of FIGS. 6A-8C, since there is no
periodicity in the radial direction, or at least since there is a
difference in cycle distance between the radial and circumferential
directions, a one-dimensional distribution of the refractive index
in the circumferential direction is created, so that a single-mode
laser oscillation can be easily obtained. It is also possible to
use the same cycle distance in both the circumferential and radial
directions, although it may result in a multi-mode oscillation.
[0067] Each of the modified refractive index areas shown in FIGS.
9A-9H has a shape created by combining one of the shapes shown in
FIGS. 5A-8C with a thin rectangle extending in the radial
direction. The presence of the rectangle further increases the
degree of asymmetry with respect to the radial axis.
[0068] An actual version of the photonic crystal laser 10 was
experimentally created to confirm its laser-oscillation capability.
The result of this experiment is hereinafter described. The
electron microscope image in FIG. 10A shows the presence of a
ring-shaped photonic crystal 123, a first groove 1241 and a second
groove 1242 in the created photonic crystal laser 10. The enlarged
image in FIG. 10B shows air holes 122.
[0069] It was confirmed that a single-wavelength laser beam of
approximately 987 nm was generated from the created photonic
crystal laser 10, as shown in FIG. 11. The laser beam had a
halo-shaped cross section as shown by the near-field image in FIG.
12A. A far-field image of the beam was a Fourier-transform image of
the halo-shaped near-field image (FIG. 12B). Additionally, to
ascertain the polarization of the laser beam, a polarizer was
placed perpendicularly to the laser beam, and far-field images of
the laser beam passing through the polarizer were taken while
rotating the polarizer on an axis perpendicular to the polarizer.
FIGS. 13A-13D show the taken images, each of which demonstrates
that, regardless of the orientation of the polarizer, the laser
beam was detected only in specific areas corresponding to the
oscillating direction of the electric field (as indicated by the
arrows in FIGS. 13A-13D) that was allowed to pass through the
polarizer. This means that the laser beam was polarized in the
radial direction.
[0070] With reference to FIGS. 14A-15D, examples of the devices
using a ring-shaped photonic crystal layer 52 having air holes
(modified refractive index areas) with different inner and outer
circumferential widths are hereinafter described. In the example
shown in FIGS. 14A and 14B, each of the air holes 522 is in the
form of a closed area defined with two half lines extending in the
radial direction from the center of the ring-shaped photonic
crystal 523 and with the inner and outer circumferences of the
ring. These areas are arranged along the circumference of the
ring-shaped photonic crystal 523. With .theta. denoting the angle
made by the two half lines as well as r.sub.1 and r.sub.2
respectively denoting the inner and outer diameters of the ring,
the inner and outer circumferential widths of the air hole 522 are
given by r.sub.1.theta. and r.sub.2.theta., respectively, where
r.sub.1.theta. is greater than r.sub.2.theta.. The configurations
of the other elements (the configurations of the base body of the
photonic crystal layer 52 and the layers other than the photonic
crystal layer 52) are the same as those of the photonic crystal
laser 10.
[0071] Shaping the air holes 522 in the previously described manner
makes their filling factor to be .theta./.phi. at any position in
the radial direction of the ring-shaped photonic crystal layer 52
(where .phi. is the angle between one of the two half lines and the
corresponding half line of the neighboring air hole). As a result,
the effective permittivity within the ring-shaped photonic crystal
will be uniform, so that the electric-field distribution of the
light will also be uniform. In the case of a rectangular air hole
with both the inner and outer circumferential widths being
r.sub.1.theta., the filling factor on the inner circumference is
.theta./.phi., while the filling factor on the outer circumference
is (r.sub.1/r.sub.2).times.(.theta./.phi.). Thus, the filling
factor on the outer circumference has a smaller value. It should be
noted that the filling factor in this paragraph is defined as the
ratio of the sections occupied by the air holes on the
circumference of a circle having a certain diameter.
[0072] In the example of FIGS. 14A and 14B, the filling factor has
the same value at any position in the radial direction. If the
outer circumferential width is greater than the inner
circumferential width and smaller than
(r.sub.2.sup.2/r.sub.1.sup.2) times the latter width (e.g. as in
FIG. 15A), the variation in the filling factor will be smaller than
in the previous case of the rectangular air hole, so that the
spatial distribution of the intensity of light will also be
smaller. If the outer circumferential width is greater than
(r.sub.2.sup.2/r.sub.1.sup.2) times the inner circumferential width
(FIG. 15B), the intensity of the laser beam on a cross section will
be higher in the inner portion of the halo. If the outer
circumferential width is smaller than the inner circumferential
width (FIG. 15C), the intensity of light will be higher in the
outer portion of the halo. The shape of the air holes is not
limited to the previously described ones. For example, the
aforementioned air hole 122, which is asymmetrically shaped with
respect to the radial axis 129, can be modified so that its inner
and outer circumferential widths differ from each other (FIG. 15D).
It is also possible to use an air-hole group consisting of a
plurality of air holes arranged in the radial direction as shown in
FIGS. 5A-9H with the inner and outer circumferential widths
differing from each other.
[0073] With reference to FIGS. 16-20, examples of the devices using
a ring-shaped photonic crystal 62 having main holes (main modified
refractive index areas) and sub holes (sub modified refractive
index areas) are hereinafter described. In the example shown in
FIG. 16, each air hole 622 consists of a main hole 622A and a sub
hole 622B which is smaller in area than the main hole 622A. The
main and sub holes are arranged side by side in the circumferential
direction of the ring-shaped photonic crystal 623. The distance
(cycle distance) between the neighboring air holes 622 is .alpha..
The distance between the main hole 622A and the sub hole 622B in
each air hole 622 is .delta.. In the present embodiment, each of
the main and sub holes 622A and 622B has a larger outer
circumferential width and a smaller inner circumferential width,
although it is possible to design these air holes so that their
inner circumferential widths are greater than the outer
circumferential width, or so that the inner and outer
circumferential widths are equal to each other. The configurations
of the other elements (the configurations of the base body and the
layers other than the photonic crystal layer) are the same as those
of the photonic crystal laser 10. FIG. 17 is an electron microscope
image showing an enlarged view of the air holes 622 in an actually
created sample of the ring-shaped photonic crystal 623.
[0074] When the main and sub holes 622A and 622B are provided in
the previously described manner, an optical path difference of
2.delta. for the light propagated in the circumferential direction
of the ring-shaped photonic crystal 62 is created between the light
reflected by one main hole 622A and the light reflected by the sub
hole 62213 paired with the aforementioned one main hole 622A in the
same air hole 622. These two rays of light either constructively or
destructively interfere with each other. Even in the case of the
destructive interference, the two rays of light will not be
completely cancelled out since the main hole 622A and the sub hole
622B have different areas.
[0075] In particular, for a light of wavelength .alpha. (i.e. the
wavelength equal to the cycle distance of the air holes 622), which
is to be amplified by the ring-shaped photonic crystal, the
interference occurs in the most destructive form when the distance
.delta. is .alpha./4. This prevents the intensity of light from
being locally strengthened on the ring. Thus, a laser light having
a halo-shaped cross section with a uniform intensity distribution
will be obtained.
[0076] FIGS. 19A and 19B show the result of a calculation of the
optical coupling coefficient .kappa. for a device with no sub holes
622B (FIG. 19A) and for a device with the sub holes (FIG. 1913). In
this calculation, the value of 6 was set at .alpha./4. The filling
factor of the sub holes 622B was fixed at 0.1, while the filling
factor of the main holes 622A was changed as a variable of the
calculation. It should be noted that the filling factor in this
calculation was defined as the ratio of the area of the air holes
to the area of the ring-shaped photonic crystal. From the result of
this calculation, it can be said that, when the sub hole 622B
having an area smaller than the main hole 622A is provided (when
the filling factor of the main hole 622A in FIG. 19B is greater
than 0.1), the optical coupling coefficient .kappa. becomes smaller
than in the case where no sub hole 622B is present, so that the
intensity of light is prevented from being locally strengthened on
the ring.
[0077] The combinations of the main and sub modified refractive
index areas are not limited to the previous ones. For example, as
shown in FIG. 22, it is possible to use a main hole 622A1 or 622A2
formed by a series of identically shaped air holes arranged in the
radial direction (where each of these air holes does not correspond
to one complete modified refractive index area but a portion
thereof) and a sub hole 622B1 or 622B2 formed by a series of air
holes arranged in the radial direction, with the diameter of these
air holes being smaller than that of the air holes forming the main
hole 622A1. The shape of the main modified refractive index area
may be different from that of the sub modified refractive index
area (e.g. one of them may be a circle and the other may be a
triangle). The main and sub modified refractive index areas may be
made of different materials having different refractive indexes
(including the case of using an air hole for one of them).
EXPLANATION OF NUMERALS
[0078] 10, 10A, 10B . . . Photonic Crystal Laser [0079] 11 . . .
Active Layer [0080] 12, 52, 62 . . . Photonic Crystal Layer [0081]
121 . . . Base Body [0082] 122, 522, 622, . . . Air Hole (Modified
Refractive Index Area) [0083] 123, 523, 623 . . . Ring-Shaped
Photonic Crystal [0084] 1241 . . . First Groove [0085] 1242 . . .
Second Groove [0086] 129 . . . Radial Axis [0087] 13 . . . Lower
Electrode Layer [0088] 131, 131A . . . Lower Electrode (First
Electrode) [0089] 132 . . . First Insulating Film [0090] 14, 14B .
. . Upper Electrode Layer [0091] 141 . . . Upper Electrode (Second
Electrode) [0092] 142 . . . Second Insulating Film [0093] 143 . . .
Window [0094] 151 . . . p-Type Cladding Layer [0095] 152 . . .
n-Type Cladding Layer [0096] 21 . . . Laser Beam [0097] 31 . . .
Current-Passing Portion [0098] 32 . . . Insulating Film [0099] 41 .
. . Ring-Shaped Lens [0100] 622A, 622A1, 622A2 . . . Main Hole
(Main Modified Refractive Index Area) [0101] 622B, 622B1, 622B2 . .
. Sub Hole (Sub Modified Refractive Index Area)
* * * * *
References